1. Field of the Invention
Embodiments of the present invention generally relate to power conversion and, more particularly, to an apparatus for extending the zero-voltage switching (ZVS) range in a DC/DC converter.
2. Description of the Related Art
A common topology for DC/DC converters is to operate a flyback converter in a quasi-resonant mode, where the primary switch is activated at the valley of the drain voltage (i.e., a minimum point in the drain-source voltage). The quasi-resonant flyback is a variation of the hard switched flyback, which utilizes the parasitic capacitance of the switch, or even an added capacitance, to absorb leakage inductance energy resulting from a leakage inductance of the DC/DC converter transformer. In addition, by adequately choosing the activation time of the switch, it is possible to have a zero-voltage switching (ZVS) activation characteristic, as well as ZVS deactivation characteristic, in order to improve overall efficiency.
One issue with such an approach is that a true ZVS transition only occurs in a limited input voltage range and cannot be achieved for all operating conditions. For example, the secondary reflected voltage has to be higher than the input voltage to have a ZVS activation. If such conditions are not met, the energy stored in the capacitance around the primary switch is wasted as the voltage across the primary switch is re-set when the switch turns on, leading to a significant loss of efficiency.
Therefore, there is a need in the art for the ability to extend the ZVS range in DC/DC converters.